The Heron’s Blink

Black-crowned Night Heron (Nycticorax nycticorax)

This Great Blue Heron rivals human mimes in its ability to stay still like a statue. However, expert human mimes do not blink, or so it is said (Source 1, Source 2). This bird blinked. When I first saw the video, I missed the blink entirely. Did you see it? To flag the event, I put a subtitle into the video during the one second in which the first blink takes place. There’s a second blink later. The actual blink motion takes only about one third of a second, judging by how many video frames it occupies. At the end of the video I’ve attached a slo-mo version of the blink. It looks like the bird is extending its nictitating membrane from front to back, covering about two thirds of the eyeball, or just enough to wipe the pupil, and then retracting it.

That’s fast work. Whether the fishes that the bird is hunting have eyes fast enough to catch that motion is unknown. One study of temporal resolution (how fast an image a fish can see) in the eyes of four common fish species found magnitudes from about 3 to about 30 Hz, meaning that they could see images lasting from about a third to about a thirtieth of a second. That would be fast enough to spot the heron’s blink. But seeing a semi-transparent light blue membrane moving across an eye the size of a pea located up in the air when you’re a fish with eyes adapted to vision in water is another matter. Chances are that the heron’s blink will spook no fish.

Here’s some still photos of the same bird from different angles.

Great Blue Heron (Ardea herodias) hunting in a spot usually reserved for the Black-crowned Night Heron
Great Blue Heron (Ardea herodias)

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5 thoughts on “The Heron’s Blink

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  • I’m definitely getting rusty. You and Peter R are right, it’s a Great Blue. I’ve corrected the post. This is a spot where I’ve repeatedly seen the Black-crowned Night Heron doing exactly what this bird is doing. Thank you for your kind words.

  • Yes, Mary, it’s a Great Blue Heron.

    There’s another half-blink at 20 seconds, as well.

    I agree –Marty’s web site is an invaluable gift to the City and to the visitors of CCP, as it continues to characterize the biodiversity –not just by naming the organisms but more significantly by detailing what those organisms are doing –acts and events that any visitor can view, and who can discover their own “firsts” too.

  • I’d have thought that was a Great Blue Heron, partly from the white above the dark head stripes and partly from the beak color. I’m only posing this because I’d like to know. I SO appreciate your fabulous photography and obvious care for the park. You’ve sharpened my interest in my walks there, and I’m very grateful.

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